1 . Field of the Invention
The subject invention relates to ball valves that permit flow rates to be predictably varied between the fully opened and the fully closed positions of the valve.
2. Description of the Prior Art
Prior art ball valves include a valve housing having an upstream end, a downstream end and a valve seat therebetween. An upstream fluid passage extends from the upstream end to the valve seat, and a downstream fluid passage extends from the valve seat to the downstream end. The upstream and downstream fluid passages are cylindrical and are axially aligned with one another. The prior art ball valve further includes structure for connecting the respective upstream and downstream ends of the valve housing to upstream and downstream pipes. For example, the exterior of the valve housing may include arrays of external threads at the respective upstream and downstream ends. Coupling nuts may be threaded with the respective arrays of external threads on the valve housing. The coupling nuts may be used to secure pipes or nipples to the respective upstream and downstream ends of the valve housing.
The prior art ball valve housing further includes a stem aperture aligned orthogonally to the fluid passageway and extending from an external region on the valve housing into the valve seat. A valve stem is rotatably mounted in the stem aperture. A spherical valving member, or ball, is rigidly mounted to the valve stem and is rotatably disposed in the valve seat. The typical prior art spherical valving member for a ball valve includes a cylindrical fluid passageway extending entirely therethrough. The cylindrical passageway has a diameter substantially equal to the inside diameter of the pipes with which the prior art ball valve is connected. Rotational forces on portions of the valve stem external of the housing are transmitted to the spherical valving member within the housing. The typical prior art ball valve include stops which enable the valve stem and the spherical valving member to rotate only 90.degree.. At one end of the 90.degree. range of movement, the cylindrical passageway through the spherical valve stem is aligned axially with the upstream and downstream passages through the valve housing to permit a full substantially unimpeded flow of fluid through the valve. At the other extreme of the 90.degree. range of motion, the cylindrical passageway through the spherical valving member is aligned orthogonally to the upstream and downstream passages through the valve housing such that fluid flow through the valve is completely blocked.
The rate of fluid flow through the prior art ball valve can be varied by rotating the valve stem and the spherical valving member to positions between the fully opened position and the fully closed position. However, the fluid flow through the prior art ball valve does not vary linearly with respect to the amount of rotation of the valve. In particular, the typical prior art ball valve will achieve only small amounts of fluid flow during the initial rotation of the valve stem from the fully closed position. Fluid flow will then increase very significantly for subsequent incremental small rotational movements. Thus, a graph having valve rotation on the horizontal axis and fluid flow rates on the vertical axis would be substantially parabolically generated. This characteristic of prior art ball valves is not a problem for the vast majority of situations where the valve is merely rotated between the fully opened position and the fully closed position. However, some applications require the fluid flow rate to be throttled periodically to some measured flow rate less than the fully opened flow rate. The non-linear relationship between valve rotation and flow rate makes such throttling difficult. Thus, for example, a 45.degree. rotation of the valve stem provides 50% of the range of movement of the valve stem, but is likely to provide a flow rate significantly different from 50%.
The prior art has included several attempts to provide a throttling ball valve. Typically these prior art throttling ball valves having included a gradually tapering fluid passageway through the spherical ball valve. The tapering passageway is disposed such that the cross-section at the upstream end of the passageway through the spherical valving member is larger than the cross-section at the downstream end of the passageway. In most such prior art attempts, the passageway is conically generated and is symmetrical about a diameter. An example of such a prior art throttling valve is shown in U.S. Pat. No. 3,542,337 to Scaramucci.
Some prior art throttling valves having included a non-symmetrical fluid passageway through the cylindrical valving member. On these valves, the upstream entrance to the tapering passageway is concentrically disposed relative to the axis of the passageway through the valve housing. The smaller downstream end of the passageway is at an off-center position relative to the passageway through the valve housing. Such a prior art valve is shown in U.S. Pat. No. 3,762,682 to Franck.
The fluid passageway through the spherical valving member of some prior art throttling ball valves have different shapes at the respective upstream and downstream ends, and the walls defining the fluid passageway taper continuously from the shape at the upstream end to the shape at the downstream end. For example, U.S. Pat. No. 3,542,338 to Scaramucci shows a throttling ball valve having a passageway with a circular cross-section at the upstream end of the spherical valving member. The walls of the fluid passageway through this prior art valve member gradually and continuously taper to define a rectangular or triangular cross-section at the downstream end of the fluid passageway through the spherical valving member.
While these prior art throttling ball valves provide a more direct relationship between valve rotation and fluid flow, there is a desire to provide improvements that achieve even more of a linear relationship between valve rotation and fluid flow. This need for a more linear relationship has been found to be particularly desirable in piping systems that tap methane gas from decaying material in buried landfill sites. In these environments, technicians monitoring the extraction of methane gas must frequently manually rotate the valve stems to achieve a selected proportion of the maximum flow rate. The non-linear relationship achieved with the prior art throttling ball valves has proved problematic.
Accordingly, it is an object of the subject invention to provide a ball valve capable of achieving a nearly linear relationship between valve rotation and fluid flow through the ball valve.
It is another object of the subject invention to provide a throttling ball valve that enables great throttling precision for use in gas flow systems, such as systems that tap methane gas from landfill sites.